The photoelectric sensor enables a non-contact detection of persons or objects, so that it is used to detect any objects to be conveyed in various automated machines or conveyer lines, and any person's approach to an automatic door or the like.
In FIG. 13 there is shown a section view of a conventional photoelectric sensor wherein a convex lens is fixed in a circular opening formed on a front side of a casing. Disposed on a focal point of the convex lens is a photoelectric element comprising a light emitting element such as a light emitting diode and a light receiving element such as a phototransistor. Fixed upon an inner surface of a casing are a light driving circuit, a signal amplifying circuit, an operation indicating lamp, a variable resistor and other parts so as to hinder an optical path between the convex lens and the photoelectric element. All of the aforesaid components are electrically connected to each other. An electric wire connected to an input terminal and an output terminal is connected to a device outside the casing.
Referring to FIG. 13, symbol A is a convex lens for light collection which is disposed on a front side of a casing B. Numeral 61 is a photoelectric element disposed on a focal point of the convex lens A. Numeral 62 is a wiring substrate, numeral 63 are a plurality of electronic parts disposed upon the wiring substrate 62 and numeral 64 is an operation indicating lamp. Such a photoelectric sensor can be installed on a certain place by means of brackets and screws.
In recent years, the structure of the photoelectric sensor has become more small-sized and more compact to meet users' requirements. Yet there are the following problems hindering the manufacture of a small-sized photoelectric sensor.
Firstly, when disposing necessary parts upon the inner wall of the casing of the photoelectric sensor, an assembler is forced to insert such tools as a driver, pincettes, soldering tool or the like into the interior of the casing. Accordingly, as the casing becomes smaller, assembling becomes more cumbersome. Thus, to make the photoelectric sensor smaller is contradictory to a better efficiency of assembling.
Secondly, since a convex lens is adapted for a light collecting means, a conically enlarged space must be formed in an optical path toward the photoelectric converting element. More particularly, in order to enhance the performance of the photoelectric sensor, it is desirable to increase the light incoming volume by enlarging a caliber of the convex lens and to minimize the influence of any outer disturbing light by extending a focal distance of the convex lens.
Therefore, it is necessary to dispose a lens having a larger caliber and a longer focal distance, so that a larger conical space must be formed. In this regard, to make the photoelectric sensor smaller is contradictory to the improvement of optical properties.
Further, one of the defects of the conventional photoelectric sensor is that it is inconvenient to handle. Since it is large-sized, it is easily touched by persons or objects, thereby an optical axis is biassed and it causes malfunction. Finally, when some dust or oily matter stucks to a body of the photoelectric sensor, cleaning is not easy.
Still further, when starting an inverter motor, the photoelectric converting element is provided with a metal evaporative plating thereupon in order to prevent any errors in operation that may be caused by electromagnetic radiating noises due to a magnet switching. This causes the light transmission rate to deteriorate.
In view of the foregoing problems, this invention has been attained. It is therefore a general object of the invention to provide a compact photoelectric sensor which brings a good fabricability, a super optical performance and a suitable operability, and is free from any influence of electromagnetic radiating noises.